In many applications, the formability, flexibility, lower processing temperatures, resistance to breakage and lighter weight of transparent plastics are features which make these materials attractive as substitutes for glass. However, plastics also have inherent drawbacks in comparison to glass. For example, poor abrasion resistance causes transparent plastics to scratch and haze, making them unsuited for glazing.
One means of making a plastic with glass-like properties has been to utilize a matrix of a highly crosslinked polymer with a finely divided silica. These materials are intrinsically hard and have been applied as coatings to the surface of plastic articles to improve abrasion resistance. Such abrasion resistant hardcoat compositions based on the photoinduced free radical polymerizations of multifunctional acrylates have been described recently in U.S. Pat. No. 5,708,048 and are also commercially available.
UV-cured plastics can be processed at higher rates and consume far less energy than thermally-cured systems. In addition, these materials can be applied to even thin plastics with low glass transition temperatures without causing distortion and warping usually encountered in processing at higher temperatures. In these compositions, silyl acrylate coupling agents undergo hydrolysis by water present in the aqueous dispersion of colloidal silica. By this reaction, the alkoxy groups are replaced by hydroxy groups which hydrogen bond to, or form covalent bonds with, the hydroxy groups present on the surface of the silica particle.
However, these materials possess drawbacks that diminish their attractiveness. The sensitivity of free radical polymerizations to inhibition by oxygen often causes a soft layer to be formed at the exposed surface. On the other hand, it is difficult to stabilize these materials against adventitious free radical polymerization in the absence of oxygen. As a result, the materials have relatively short shelf lives and tend to gel on standing.
Cationic photopolymerizations have the advantage that they display no appreciable oxygen inhibition and in addition, using this approach, a wide variety of different monomer systems, including epoxies, vinyl ethers, and oxetanes may be used. However, until now, it has not been possible to prepare silica-polymer composites having glass-like properties by cationic photopolymerization. There are several reasons for this. First, cationic photoinitiators are not useful with acrylate monomers. Second, a process for preparing composites based on acrylate polymers entails surface modification of colloidal silica with acrylate groups by hydrolysis and condensation of acrylate-functional alkoxysilane coupling agents in order to incorporate the silica into the polymer matrix. However, conditions required for the hydrolysis have prevented use of cationically polymerized epoxies: acid-catalyzed hydrolysis results in opening of the epoxy ring while use of basic catalysts causes inhibition of the cationic polymerization.
It is therefore an objective of this invention to provide a polymer composite which possesses glass-like properties. It is a further objective to retain the advantages of UV curing: a system which is solvent-free, low energy, and cures rapidly at low temperatures. It is still a further objective to overcome the difficulties inherent in using free radical initiators for polymerization.